Parvovirus and papovavirus replicons are being used as probes for the analysis of eukaryotic DNA replication and postreplication repair. The prototype linear DNA replicon is the parvovirus H-1 replicative form DNA and the mechanism of its replication is being analyzed using conditional mutants and defective genomes. Polyoma and SV40 DNA are being used as examples of circular DNA replicons and the interactions of these DNAs with that of parvovirus DNA are being analyzed in doubly infected cells.